This invention relates to epoxy resin compositions containing compounds which inhibit the cure of the epoxy resins at lower temperatures. The invention further relates to compositions useful for curing epoxy resins which comprise curing agents for the epoxy resins and the inhibitor described above. The invention further relates to coatings prepared from such inhibited resin compositions, and to materials coated with or encapsulated in such inhibited resin compositions. The invention further relates to laminates, prepregs, encapsulated materials and composites prepared from such inhibited resin compositions.
Epoxy resins are used in a variety of applications, as powder coatings, as electrical encapsulates, in composites, in solution coatings, in prepregs and in laminates. In many of these uses, it is desirable to partially cure the epoxy resin in a controllable manner, such that at a later time, when desired, the epoxy resin can be fully cured. In many uses it is desirable to contact a curing agent for an epoxy resin with an epoxy resin in the presence of a catalyst for the reaction and have the composition remain stable, that is uncured for a period of time. At present it is quite difficult to control the curing of an epoxy resin and retain the stability of a composition comprising an epoxy resin, a curing agent and catalyst for the reaction of the epoxy resin with the curing agent.
In some uses the epoxy resin, curing agent, and catalyst are contacted in solution and then applied as a coating on a substrate. In such uses, in order to achieve optimal properties in the eventually cured epoxy resin, the solvent or solvents must be removed. Often it is desirable to remove these solvents before significant curing takes place, otherwise the solvent may be entrapped in the cured epoxy resin, thereby adversely affecting the final properties of the cured resin. Often, the solvent is removed by exposing the coated article to elevated temperatures. At such temperatures the epoxy resin may have a tendency to begin to cure and entrap the solvent, thereby adversely affecting the final properties.
Cured and partially cured epoxy resins are used in laminates. A common process used in preparing glass laminates involves passing a glass cloth to be coated with an epoxy resin composition through a bath containing the resin, a curing agent for the resin and a catalyst for the curing reaction. Generally the bath contains one or more organic solvents in which the various components are dissolved or dispersed, with a solids contents of between about 45 and about 90 percent. The glass is resident in the bath for time sufficient to coat it with a mixture of the resin, curing agent and catalyst for the curing reaction. Thereafter the coated glass is passed through a heated zone to remove remaining solvent by evaporation. Generally, this zone is at a temperature of between 120.degree. and 200.degree. C. The residence time in this zone is sufficient to allow the solvents to be completely volatilized away. Thereafter the glass cloth, coated with the resin, curing agent and catalyst from which the solvents have been removed, may be controllably partially cured by exposing it to temperatures at which curing occurs. Such a product is referred to as a prepreg. The prepregs may then be stacked or formed into a shape and exposed to conditions under which the curing agent and resin completes the cure. In general, this involves contacting the various components of a laminate at elevated temperatures under pressure, for a period of time sufficient for the epoxy resin to further cure. Usually the pressure is provided by some kind of a press. Once the laminate is removed from the press it may, optionally, be exposed to elevated temperature for a period of time to complete the curing reaction. In this curing process the resin coating on the glass cloth flows and mixes with the coating on adjacent glass clothes thereby resulting in a fusing of the glass layers together, via the cured epoxy resin.
High glass transition temperatures are desirable for many uses, therefore methods of increasing the glass transition temperatures are desired. It is further desired to significantly decrease the time necessary to achieve complete cure. Prepreg producers and laminators desire a composition which facilitates faster processing. This allows the more efficient utilization of processing equipment. In order to facilitate faster processing the coated substrates must be exposed to higher temperatures or the epoxy resin compositions must contain higher levels of catalysts. Unfortunately, both solutions result in less control over the curing reaction and solvent can be trapped in the final product thereby affecting the ultimate properties.
In one embodiment, a process referred to as continuous pressing technology is used. In this process the prepregs are contacted at much higher temperatures than in conventional laminate processing for shorter periods, e.g. at temperatures around 210.degree. C. for 2 to 4 minutes. It is very difficult to achieve complete cure under such conditions. One potential solution to this is to add a larger amount of catalyst or accelerator to the epoxy resin. Unfortunately if additional catalyst is added in the coating bath, then the ability to remove the solvent without significant curing of the epoxy resin, or to controllably partially cure the epoxy resin, is significantly reduced.
What is needed is an epoxy resin composition that does not undergo significant cure at temperatures at which solvents may be removed. What is further needed is an epoxy resin that can be controllably B-staged or partially advanced. What is further needed is an epoxy resin composition which allows the use of higher concentrations of curing catalysts or accelerators, which are stable at temperatures at which solvent is removed, and which allow controllable B-staging or partial advancement. What is further needed is an epoxy resin composition which when coated on a substrate can be processed faster without hurting the ultimate properties.
Frequently, for many products prepared using epoxy resins and cured epoxy resins several different entities may perform different parts of the manufacturing process. For example, one entity may make the resin, a second entity may make the resin formulations used to impregnate the reinforcing material, and a third may make a prepreg, or other article to be used. While a forth would make the final product such as a laminate or printed circuit board. Frequently the entity producing the prepreg or laminate has no expertise or desire to make the formulation. Therefore, it is desirable that a formulator be able to make a composition useful in coating the materials to be laminated. The problem is that if the epoxy resin curing agent and catalyst are preformulated, the formulation may not have significant longterm storage stability. Under such circumstances the formulation may undergo curing and therefore not be useful to the prepreg or laminate manufacturer. What is further needed is an epoxy resin composition containing a curing agent and accelerator for the cure which has significant stability at ambient temperatures for several weeks.